Spectroscopic Characterization of Films Obtained in Pulsed Radio

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32 Spectroscopic Characterization of Films Obtained in Pulsed Radio-Frequency Plasma Discharges of Fluorocarbon Monomers Charles R. Savage , Richard B. Timmons* , and Jacob W. Lin 1

1

2

Department of Chemistry, Box 19065, The University of Texas at Arlington, Arlington, T X 76019-0065 Polytronix Inc., 805 Alpha Drive, Richardson, T X 75081 1

2

The structure of plasma-polymerized films obtained using pulsed radio-frequency (rf) duty cycles is shown to vary significantly with the type of rf duty cycle employed. The controllability and tailoring of surface compositions as functions of the rf duty cycle are illustrated for processes carried out with two fluorocarbon monomers, namely, hexafluoropropylene oxide (C F6O) and hexafluoropropene (C F ). The variations in surface composition with rf duty cycles are documented via X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses of the films obtained. Progressive and substantial changes in the molecular composition of the plasma-deposited films as functions of the rf duty cycles employed were achieved with both monomers. The experimental results reveal a relatively high level of compositional control of polymers in the plasma polymerization of these monomers as a function of the rf duty cycle employed. In general, there is a progressive increase in the extent of polymer cross-linking with increasing rf duty cycle as evidenced by spectroscopic characterization of the films obtained in this study. 3

THE

3

6

U S E O F P L A S M A T E C H N I Q U E S T O INITIATE POLYMERIZATION PROCESSES

continues to exhibit r a p i d growth. T h e magnitude o f the current activity i n *Corresponding author 0065-2393/93/0236-0745$07.00/0 © 1993 American Chemical Society

Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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STRUCTURE-PROPERTY RELATIONS IN POLYMERS

plasma polymerization was illustrated i n terms o f the scope a n d diversity o f topics presented at the recent S y m p o s i u m o n P l a s m a P o l y m e r i z a t i o n a n d P l a s m a Interactions w i t h P o l y m e r i c Materials ( I ) . A s demonstrated at that symposium, the focus o f the majority o f recent studies lies i n the application o f plasma polymerization methods to achieve surface modifications. These modifications are obtained b y t h i n - f i l m deposition or reactive interactions o f plasma-generated intermediates w i t h surface atoms a n d molecules. T h e use o f plasma polymerization as a surface-modification technique offers a n u m b e r o f advantages over various conventional coating procedures. A m o n g these advantages are the fact that an unusually w i d e range o f monomers are available (e.g., even C H can function effectively as a " m o n o m e r " i n plasma polymerization), relatively pinhole-free films can be de posited, film thickness is usually controllable over the important range f r o m tens to hundreds o f angstroms, a n d surface modifications are achieved i n a simple a n d relatively inexpensive one-step deposition process. Yasuda (2) has p r o v i d e d a detailed description a n d analysis o f the important facets o f plasma polymerization reactions.


4

T o date, the o v e r w h e l m i n g majority o f plasma polymerization investiga tions have i n v o l v e d continuous-wave ( C W ) plasma operation, invariably at the radio frequency (rf) o f 13.56 M H z . I n general, experimentation w i t h i m p o r tant plasma variables (e.g., absorbed r f power, m o n o m e b flow velocity a n d pressure, etc.) permits identification o f reaction conditions i n w h i c h u n i f o r m films can be obtained f r o m a given m o n o m e r . A g a i n , Yasuda (2) has p r o v i d e d an excellent analysis a n d discussion o f the influence o f various plasma variables o n the dynamics o f the polymerization process a n d the quality o f the resultant films. A n important aspect of C W plasma polymerizations is the extensive frag mentation o f the m o n o m e r that occurs u n d e r C W conditions. Indeed, it is this extensive fragmentation that permits saturated molecules such as C H a n d C H to be e m p l o y e d as monomers i n these systems. A t the same time, to a relative lack o f controllability i n the molecular composition o f the films that are obtained. F o r example, the relative indiscriminate reorganization o f atoms u n d e r C W plasma polymerization conditions was aptly demonstrated i n a recent C W study o f the plasma polymerization of several carbonyl-containi n g monomers. Plasma polymerization o f acetaldehyde, acetone, a n d 2butanone y i e l d e d remarkably similar surface films despite variations i n the molecular structures a n d c a r b o n - h y d r o g e n - o x y g e n content o f these molecules (3). I n a similar v e i n , it is often n o t e d that C W plasma p o l y m e r i z a tion o f certain monomers results i n molecular compositions that frequently bear relatively little resemblance to those obtained i n the conventional polymerization o f a reactive m o n o m e r [e.g., acrylonitrile ( 4 ) and tetrafluoroethylene (5)]. T o a certain extent, r e d u c t i o n o f the absorbed r f p o w e r can, 4

2

6


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generate a closer similarity between the plasma a n d the conventionally p o l y m e r i z e d m o n o m e r as demonstrated, for example, w i t h styrene ( 6 ) . H o w ever, control of plasma-deposited film composition v i a absorbed r f p o w e r has several inherent limitations. These limitations i n c l u d e the fact that a m i n i m u m absorbed r f p o w e r is r e q u i r e d to ignite a n d maintain the plasma a n d u n i f o r m films (i.e., those d e v o i d o f p o w d e r or oillike deposits) are typically obtained only over relatively restricted p o w e r regimes for a given m o n o m e r u n d e r specified flow conditions. A d d i t i o n a l approaches to controlling film compositions i n C W plasma polymerizations i n c l u d e depositions w i t h r e d u c e d substrate temperatures ( 7 ) a n d variations i n the positions o f the substrates located downstream o f the plasma ( 8 ) . T h e purpose o f the present study was to introduce an a d d e d d i m e n s i o n to the controllability o f plasma-achieved surface modifications w i t h respect to the molecular composition o f the deposited films. W e h o p e d this a d d e d controllability c o u l d be achieved v i a p u l s e d r f plasma discharges as opposed to the standard C W operation. T h e rationale b e h i n d this approach centered o n a recognition that reactive intermediates (i.e., radicals a n d ions) p r o d u c e d d u r i n g the p l a s m a - " o n " periods may undergo decay mechanisms d u r i n g the plasma-"off " p e r i o d that are significantly different f r o m decay mechanisms observed u n d e r typical C W conditions. F o r example, relatively h i g h reactive intermediate concentrations d u r i n g the plasma-on periods t e n d to enhance the importance o f recombination or disproportionation reactions that are second order w i t h respect to reactive intermediates. O n the other h a n d , at some point d u r i n g the off p e r i o d , first-order reactions i n reactive i n t e r m e d i ates w i l l be dominant as the concentration of these reactive species drops. T h u s , competitive processes of the type Rj + R R

x

—> products

(1)

( o r R ) + M -> products

(2)

2

2

where R a n d R represent reactive intermediates and M is a m o n o m e r , c o m p a r e d to reaction 2 u n d e r plasma-on c o m p a r e d to plasma-off conditions. F u r t h e r m o r e , i f such effects are i n d e e d noted, the actual molecular c o m p o s i tions o f the polymers that are obtained might be tunable to a certain extent w i t h respect to the type of r f duty cycle e m p l o y e d (i.e., the ratio of o n time to o n + off times). I n connection w i t h this idea, it is significant to note that the l i m i t e d literature data that c o m p a r e d fixed duty cycle p u l s e d a n d C W plasma polymerizations support the fact that m a r k e d differences i n film growth rates a n d residual radical concentrations are observed for selected monomers ( 2 , plasmas (JO). x

2



748


This chapter reports o n a detailed study o f variable duty cycle p u l s e d r f plasma polymerizations as a route to achieving a higher level o f molecular compositional control o f the plasma-initiated surface modifications obtained. T h i s topic is explored h e r e i n w i t h respect to systematic variations i n r f duty cycles e m p l o y e d i n the plasma polymerization o f several monomers, w h i l e all other plasma variables are kept constant. T h e molecular compositions o f the p o l y m e r films obtained as functions o f r f duty cycle were m o n i t o r e d spectroscopically using X - r a y photoelectron spectroscopy ( X P S ) a n d F o u r i e r trans f o r m i n f r a r e d ( F T I R ) spectroscopy. T h e combination o f these spectroscopic approaches provides clear a n d abundant evidence for significant variations i n the molecular composition o f the plasma-deposited films as functions o f the r f duty cycles e m p l o y e d . These initial results are encouraging w i t h respect to achieving controllability o f the molecular composition o f the surface m o d i f i cations obtained i n plasma polymerizations. Some o f the implications a n d potential applications o f such surface molecular compositional control are considered briefly at the e n d o f this chapter.

Experimental Details A l l plasma polymerizations were carried out i n a cylindrical Pyrex glass reactor that was 10 c m i n diameter a n d 30.5 c m i n length. Radio-frequency p o w e r was p r o v i d e d to this reactor b y two concentric metal rings located at either e n d o f the reactor. T h e reaction system a n d associated electronics are shown i n F i g u r e 1. T h e r f circuit i n c l u d e d a f u n c t i o n generator (Wavetech M o d e l 166), a pulse generator (Tetronik M o d e l 2101), a r f amplifier ( E N T M o d e l A300), a frequency counter ( H P M o d e l 5381A), a wattmeter ( B i r d ; to measure absorbed a n d reflected power), a n d a c a p a c i t o r - i n d u c t o r m a t c h i n g network used to tune the circuit to m i n i m i z e reflected power. A n oscillo scope, previously calibrated against the wattmeter u n d e r C W conditions, was e m p l o y e d i n t u n i n g the circuit to m i n i m i z e reflected p o w e r u n d e r r f p u l s e d operation. A l l data reported i n this study represent runs carried out at a r f frequency o f 13.56 M H z . A n M K S butterfly valve controller (Baratron M o d e l 2 5 2 A ) was used to b o t h m o n i t o r a n d control pressure i n the reactor. A standardized procedure was adopted for all film depositions. Substrates that consisted o f p o l i s h e d S i a n d K C l disks were p l a c e d o n top o f an inverted 2 0 - m L beaker located i n the center o f the reaction chamber. T h e S i samples were cleaned v i a sonification a n d rinsing p r i o r to placement i n the reactor. T h e system was then evacuated to a pressure ^ 1 μπι, after w h i c h the sample was subjected to a 1 0 - m i n C W argon plasma at an A r pressure o f 0.7 torr a n d r f absorbed p o w e r o f 200 W . T h i s A r pretreatment was c o n d u c t e d to ensure surface cleanliness o f the substrates. Subsequently, the A r flow was terminated a n d the reactor system was evacuated to background pressure before admission o f the reactant monomers. B e t w e e n runs the reaction


Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993. Matching Network

—12cm—

Forward Power Oscilloscope

Bidirectional Coupler

To Vacuum Pump

Sample

Reflected Power

Watt Meter

M

Pulse Generator

RF Gene rator

RF Amplifier

Figure 1. Schematic diagram of the pulsed rf plasma components and the flow-tube reactor.

Faraday Cage

To Baratron Gauge

Butterfly Valve Flow Controller


750


chamber was cleaned using an 0 the c h a m b e r walls.

2

plasma to remove surface depositions o n

Because the focus o f this study was the investigation o f the effect o f r f duty cycle o n film compositions obtained i n p u l s e d plasma polymerizations, p o w e r a n d constant m o n o m e r pressure a n d flow velocity. I n this way, w e were able to isolate r f duty cycle effects o n the c h e m i c a l composition o f the films obtained. T h e actual reaction variables e m p l o y e d w i t h each o f the monomers studied are identified i n the following text. Hexafluoropropylene ( C F ) a n d hexafluoropropylene oxide ( C F 0 ) were e m p l o y e d as monomers i n this study. T h e gaseous C F and C F O reactants were obtained f r o m P C R Inc. and they h a d stated p u r i t y i n excess of 9 8 % , w h i c h was c o n f i r m e d b y gas chromatography-mass spectroscopy analysis. 3

6

3


3

6

3

6

e

T h e X P S spectroscopic characterizations w e r e carried out using a spec trometer ( P e r k i n - E l m e r P H I 5000) e q u i p p e d w i t h an X - r a y source monochromator. T h e X - r a y source was A l K = 1486.6 e V . A pass energy o f 17.90 e V giving a resolution o f 0.60 e V w i t h A g (3d ) was e m p l o y e d . T h e X P S results of fluorine-containing monomers ( C F O and C F ) were shifted (i.e., standardized) b y centering the F ( I s ) peak at 689 e V . A n electron flood gun was e m p l o y e d to neutralize charge b u i l d u p o n the insulator-type films involved i n this work. T h e electron flood g u n was operated u n d e r conditions that p r o v i d e d o p t i m u m resolution o f the C ( I s ) peaks. T y p i c a l conditions for the neutrafizer were 21.0-ms emission current a n d 3.0-eV electron energy. A

5/2

3

e

3

6

F i l m deposition rates were d e t e r m i n e d b y measurement of the film thicknesses using a profilometer (Tencor A l p h a Step 200) after each r u n . A d i a m o n d - t i p p e d p e n was e m p l o y e d to scribe a t h i n scratch o n the plasmadeposited films, after w h i c h the film thicknesses were d e t e r m i n e d . S i l i c o n substrates were e m p l o y e d i n the film-thickness measurements.

Results Substantial changes i n the molecular composition of plasma p o l y m e r i z e d films w e r e n o t e d as a function o f e m p l o y e d r f duty cycles for b o t h o f the monomers. These results are presented w i t h respect to each o f the reactants i n the following sections.

Hexafluoropropylene Oxide. Plasma polymerizations o f hexafluo ropropylene oxide ( C F 0 ) were carried out at a m o n o m e r pressure o f 0.43 torr, a flow velocity o f 9.6 c m / m i n (standard temperature a n d pressure), a n d an absorbed r f p o w e r o f 300 W . Polymerizations were carried out over a w i d e range o f r f duty cycles. I n one set o f experiments, a constant r f o n time (e.g., 3

6

3


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l o n g as 1000 ms. I n a second set o f experiments, a constant off time (i.e., 200 ms) was e m p l o y e d w h i l e the o n t i m e was v a r i e d systematically over the range f r o m 10 to 70 ms. Additionally, several other o n - o f f ratios were studied as identified i n the following paragraph. I n b o t h experimental sequences (i.e., constant o n - v a r i a b l e off t i m e a n d constant o f f - v a r i a b l e o n time), m a r k e d changes i n the molecular composition of the plasma-deposited films were noted w i t h variation o f the r f duty cycle. T h e compositional changes are d o c u m e n t e d b y b o t h X P S a n d F T I R analyses o f these films. T h e X P S results are s u m m a r i z e d i n Figures 2 - 4 . E a c h o f these spectra shows high-resolution scans o f the C ( I s ) b i n d i n g energy. F i g u r e 2 depicts the variation of the C ( I s ) region obtained for films deposited at a constant plasma-on t i m e o f 10 ms as a f u n c t i o n o f a progressively longer off t i m e that corresponded to values o f 20, 60, 100, a n d 1000 ms (curves a ~ d , peaks i n these spectra can be assigned to the following functional groups (11, e V . Plasma p o l y m e r i z e d fluorocarbon films generally exhibit four p r o m i n e n t peaks i n that only a single b r o a d peak centered i n the 2 9 0 - 2 8 9 - e V b i n d i n g region is observed. T h i s observed peak corresponds to unresolved C F - s u b stituted a n d nonfluorine-substituted groups. Because w e are interested i n the correlation o f trends i n group functionalities w i t h varying r f duty cycles, the X P S spectra are interpreted i n terms o f C F , C F , C F , a n d C groups. T h e C group refers to C b o u n d to C F and C F . A d d i t i o n a l l y a C ( I s ) peak at ~ 285 e V (observed only i n the 10 ms o n - 2 0 ms off a n d C W runs) is typical o f a graphitic or S i C carbide structure. Analysis o f F i g u r e 2 reveals a clear shift i n film c o m p o s i t i o n w i t h increasing o f f t i m e t o w a r d a progressively m o r e dominant C F - t y p e structure. 3

3

2

2

2

T h e second set o f experiments, c a r r i e d out w i t h a constant off t i m e o f 200 ms a n d systematic variation o f the o n t i m e over the interval o f 10, 20, 30, as functions o f r f duty cycle. T h e s e results are presented i n F i g u r e 3 i n terms o f two stacked plots i n w h i c h each spectrum was n o r m a l i z e d w i t h respect to the peak heights o f the C F carbons. Analysis o f F i g u r e 3 reveals a clear, C F , C F , a n d C functionalities w i t h increasing plasma-on times. 3

3

Finally, a series o f runs carried out at a constant plasma o n - o f f ratio o f 1/20 b u t w i t h varying o n - o f f pulse times p r o d u c e d the C ( I s ) X P S results shown i n F i g u r e 4. A g a i n , dramatic changes i n the relative p r o p o r t i o n o f C F carbon atom functionalities are noted over the o n - o f f sequence o f 1/20, observable coating as shown b y X P S analysis. T h e X P S spectra for the 10/200, 20/400, a n d 100/2000 runs are shown i n F i g u r e 4. T h e molecular compositional changes achieved i n film structure as functions o f the r f duty cycle e m p l o y e d i n these plasma polymerizations are


752


further c o n f i r m e d b y F T I R analysis. A n example o f these changes is shown i n F i g u r e 5 for a series o f plasma polymerizations carried out at r f duty cycles o f o n - o f f ratios ( i n milliseconds) o f 10/20, 10/200, a n d 10/400 (curves a, b , teristic o f C F stretching vibrations. A s shown i n this

figure,

the b r o a d ,

o n - 2 0 m s o f f d u t y cycle is replaced w i t h a clear doublet structure as the o f f portion o f the duty cycle is increased. This well-resolved doublet, indicative


o f a highly o r d e r e d solid-state structure, is remarkable w h e n c o m p a r e d w i t h

I 296

I 294

I 292

I 290

I 288

I 286

Figure 2. High-resolution C (is) XPS results obtained with C F 0 showing the change in molecular structure of the plasma-deposited surface films as a function of the pulsed rf duty cycle employed. Curves a~d represent rf on-off duty cycles of10/20, 10/60, 10/100, and 10/1000 (in ms), respectively. 3

6


32.

SAVAGE ET AL


753


10r

Φ

10r

>

h

Binding Energy, eV Figure 2. Continued

typical infrared ( I R ) absorption spectra o f fluorocarbons obtained u n d e r C W plasma polymerization conditions ( 2 ) . T h e frequencies o f these two doublets are at 1207 a n d 1153 c m . I n light o f the aforementioned X P S C (Is) results for 10 ms o n - v a r i a b l e o f f time ( F i g u r e 2), it seems reasonable to assign the 1 1 5 3 - c m " b a n d to a C F stretch associated w i t h C F groups, because it is clearly the C F functionality that grows w i t h increasing o f f times. F u r t h e r more, it is significant to note that w e observe a constancy i n the ratio o f the 1 1 5 3 - 1 2 0 7 - c m peaks at o f f times greater than 400 ms. This constancy i n film molecular composition at very long r f o f f times is c o n f i r m e d b y the X P S - 1

1

2

2

- 1



754


282.0

280.0

Binding Energy, eV Figure 3. High-resolution C (is) XPS results obtained with C F O showing the progressive variation in plasma-deposited surface films for runs at constant off time of200 ms and on times of 10, 20, 30, 40, and 70 ms are shown. Plots were normalized with respect to the high binding energy 294-eV peak (CF ). 3

e

3

analysis, w h i c h reveals virtually n o change i n composition over the o n - o f f sequences

( i n milliseconds) o f 10/400, 10/600, 10/800, 10/1000, a n d

10/1200. T h e X P S a n d F T I R spectroscopic characterizations reveal that variation o f the r f duty cycle i n the plasma p o l y m e r i z a t i o n o f C F 0 results i n clear 3

6

changes i n the surface film compositions obtained. T h e characteristic feature o f these changes is that a decrease i n the ratio o f o n - o f f times results i n less cross-finking of the films obtained (i.e., a decrease i n C a n d C F groups w i t h



32.

SAVAGE ET AL

755

Fulsed Radio-Frequency Plasma Discharges

i

1

i

295.0

i

I

290.0

285.0

Binding Energy, eV Figure 4. High-resolution C (is) XPS results with C F 0 obtained at a constant on-qff duty-cycle ratio of 1/20 but with variable on-off pulse widths of 10/200, 20/400, and 100/2000 (in milliseconds) as shown. Plots were normalized as in Figure 2. 3

6

progressive growth o f the C F functionality). I n fact, the films obtained at duty cycles involving relatively small o n - o f f plasma duty-cycle ratios a n d sufficiently l o n g o f f times (i.e., longer than 100 ms) are consistent w i t h the formation o f a highly linear C F polymer, w h i c h is relatively similar to that obtained i n the conventional polymerization o f C F to f o r m poly(tetrafluoroethylene) (Teflon). A l s o note that the variation f r o m a cross-linked to a linear p o l y m e r w i t h decreasing r f duty cycle is c o n f i r m e d b y physical proper ties o f the films obtained. F o r example, there is a noticeable decrease i n film hardness w i t h decreasing r f duty cycle. 2

2

2

4

Overall, as d o c u m e n t e d i n Figures 2 through 5, the variation o f r f duty cycle i n the p u l s e d plasma polymerization o f this m o n o m e r provides a means to control tunability o f the molecular composition o f the plasma-produced polymers. I n the case o f C F O reactant, this surface-composition tunability encompasses very w i d e changes i n the relative proportions o f the c a r b o n fluorine group functionalities p r o d u c e d . A n important feature o f the dynamics o f C F 0 p u l s e d plasma p o l y m e r ization is n o t e d w i t h respect to film growth rate as a function o f r f duty cycle. T h i s film growth rate is useful to t h e understanding o f the variation i n plasma-produced film compositions as functions o f r f duty cycle e m p l o y e d . A n interesting example o f the film growth r a t e - r f duty cycle relationship is shown i n F i g u r e 6. I n this diagram, the film deposition rate (expressed i n 3

g

3

6



756


Wavenumbers (cm* ) 1

Figure 5. FTIR absorption spectra obtained from the plasma deposition of C F 0 as a function of the rf duty cycle employed. The curves represent data obtained for rf duty cycles of 10/20, 10/200, and 10/40 (in milliseconds) as shown in curves a, b, and c, respectively. 3

6

milliangstroms p e r Joule absorbed r f energy) is plotted as a function o f the r f off time f o r a series o f runs carried out w i t h a constant o n time o f 10 ms. T h e film-deposition rate is striking i n that a r a p i d increase i n film growth rate is noted w i t h increasing o f f time. A s this result clearly demonstrates, significant polymerization that leads to film deposition occurs d u r i n g the o f f p o r t i o n o f the r f duty cycle. I n fact, note that u n d e r the reaction conditions e m p l o y e d i n this study, n o film deposition was obtained w h e n the plasma was operated u n d e r a C W condition. T h e lack o f C W film deposition is i m p l i e d b y the data shown i n F i g u r e 6. A s shown i n F i g u r e 7, a plot o f film deposition rate as a function o f plasma-on time at a constant o f f time o f 200 ms is also revealing. I n this series, the deposition rate goes t h r o u g h a sharp m a x i m u m at 10 ms o n - 2 0 0 ms o f f as the r f duty cycle is increased at a constant o f f p e r i o d o f 200 ms. T h e results s h o w n i n Figures 6 a n d 7 are clearly supportive o f major differences i n film deposition rates d u r i n g o n a n d o f f periods o f the p u l s e d plasma polymerization. W e believe that the r a p i d increase i n film deposition


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Plasma Discharges


400

1200

OFF TIME (ms) Figure 6. Film deposition rate obtained during the pulsed rf plasma polymerization of C F 0 as a function of the plasma-off duration with a constant on time of 10 ms. The deposition rate is expressed in terms of milliangstroms per Joule of absorbed rf energy. 3

6

rate d u r i n g the plasma-off periods ( F i g u r e 6), c o u p l e d w i t h the simultaneous growth i n C F functionality i n the p o l y m e r films w i t h increasing o f f periods (cf. F i g u r e 2) is indicative o f a free-radical-chain process that is operative i n this system. W e speculate that reactive intermediates R (i.e., ions o r radicals) that are p r o d u c e d d u r i n g the plasma-on periods can initiate a reaction o f the type 2

F

F

I

I

R + CF — C

C— F

3

>

RCF CF 2

· + CF20

2

(3)

Ο by reaction w i t h undissociated C F O m o n o m e r . Subsequently, the R C F C F · radical p r o d u c e d i n reaction 3 can continue the polymerization process b y the reaction 3

2

e

2

F

F

I

I

RCF CF - + CF — C 2

2

C— F

3

>

R(CF ) CF - + CF20 2

3

2

Ο


(4)

758



100

0

10

20

30

40

50

ON Time (ms) Figure 7. Film deposition rate obtained during the puked rf plasma polymerization of C F O for runs with a constant off time of200 ms expressed as a function of the on times employed. 3

e

T h e c o n t i n u e d reaction o f R ( C F ) C F · w i t h the m o n o m e r represents a chain reaction that leads to the formation of p o l y m e r i c R ( C F ) C F · units and a resultant C F - t y p e linear polymer. It is important to note that C F 0 is an exceptionally stable species [ A G f ( C F 0 ) = —624 k j / m o l where G f is the fracture energy] at 298 Κ (13), w h i c h makes reactions of types 3 and 4 favorable processes i n terms o f free-energy considerations. This type of chain growth is strongly supported b y the complete absence o f oxygen (as shown b y b o t h X P S a n d F T I R analysis) i n all films obtained f r o m C F O polymeriza tion. 2

3

2

2

n

2

2

2

2

3

e

A l t h o u g h the foregoing radical-chain process c o u l d also be operative d u r i n g plasma-on periods, the higher free-radical concentrations present d u r i n g o n periods w i l l favor radical-radical-type processes that lead to a relative loss i n product specificity a n d thus more highly cross-linked struc tures as observed at higher o n - o f f r f duty cycle ratios ( F i g u r e 3). I n addition to the aforementioned film-forming processes, the experi mental evidence is also strongly suggestive of the presence of ablation i n d u c e d a n d plasma-on i n d u c e d surface rearrangement processes that are competitive w i t h the film deposition step. F o r example, as noted previously,


32.

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759

C W conditions. F u r t h e r m o r e , polymerizations carried out at a fixed plasmao n - p l a s m a - o f f ratio o f 1/20 a n d varying pulse lengths resulted i n a signifi cant variation i n the p r o p o r t i o n o f C F groups as shown i n F i g u r e 4. I n this figure, a sharp d i m i n u t i o n o f C F groups is noted w i t h increasing o n times and fixed o n - o f f ratios. T h i s systematic change i n film composition can b e rationalized i n terms o f p l a s m a - i n d u c e d molecular rearrangements that result f r o m impact o f high-energy species w i t h the initially deposited surface films. Alternatively, the results i n F i g u r e 4 might b e indicative o f preferential ablation o f C F groups d u r i n g the plasma-on p e r i o d . Because these changes occur at a fixed o n - o f f ratio o f 1/20, it seems clear that some type o f inductive (i.e., time-delay) process is involved i n the creation o f the reactive species responsible f o r the plasma-induced molecular rearrangement or abla tion processes. T h e occurrence o f a time-delayed ablation process d u r i n g the plasma-on p e r i o d is also strongly c o n f i r m e d b y the deposition rate data shown i n F i g u r e 7 i n w h i c h a sharp decrease i n the rate o f film formation is noted at increasing o n times longer than 10 ms. 2


2

Perfluoropropylene. Plasma polymerizations were carried out using perfluoropropylene ( C F ) at a m o n o m e r pressure o f 0.43 torr a n d a flow velocity o f 9.6 c m / n i m (standard temperature a n d pressure). I n contrast to the 3 0 0 - W r f p o w e r u s e d i n the C F O studies, the r f p o w e r e m p l o y e d i n the majority o f runs was 200 W . T h i s lower p o w e r was e m p l o y e d w i t h C F because it p r o d u c e d better quality films at higher deposition rates. 3

6

3

3

e

3

6

Systematic studies o f the effect o f variations i n r f duty cycle o n the plasma polymerization o f C F were carried out w i t h a l l other reaction variables kept constant. A s i n the case o f C F O , two time sequences were used to obtain film depositions: a sequence o f a constant o n time a n d variable off time as w e l l as a series o f experiments w i t h a constant o f f time a n d variable o n time. I n addition, experiments at a fixed o n - o f f ratio o f 1/2 b u t w i t h variable o n - o f f pulse widths were also carried out. 3

6

3

e

X P S analyses o f the C (Is) region o f the plasma p o l y m e r i z e d films that were obtained w i t h this m o n o m e r are shown i n Figures 8 - 1 0 . F i g u r e 8 summarizes results for a constant o n time o f 10 ms a n d o f f periods o f 20, 100, the C F group peak heights. A s revealed i n this figure, there is a m a r k e d increase i n the relative p r o p o r t i o n of C F groups (vis â vis other C F a n d C groups) w i t h increasing o f f times. T h i s behavior parallels that observed w i t h C F 0 m o n o m e r (cf. F i g u r e 2). T h e C (Is) X P S spectra for films obtained at constant off a n d variable o n times are shown i n F i g u r e 9. I n this series, 200 ms are shown as curves a, b , a n d c, respectively. A g a i n , the results parallel those f r o m C F O (cf. F i g u r e 3) i n w h i c h the C F molecular fraction decreases substantially a n d systematically as the o n time increases. Finally, 3

2

3

6

3

e

2



300.0

290.0

Binding Energy, eV

295.0

3

285.0

3

6

Figure 8. High-resolution C (is) XPS results obtained with C F showing the molecular structure variation of the plasma-deposited surface films as a function of the rf duty cycle employed. Curves shown represent data obtained for rf on-off duty cycles on 10/20, 10/100, and 10/200 (in milliseconds), as indicated. Plots were normalized with respect to the high binding energy 294-eVpeak (CF ).

H


32.

SAVAGE ET AL

761



10

Binding Energy, eV Figure 9. High-resolution C (is) XPS results obtained with C F showing the variation in molecular structure of films deposited during pulsed rf plasma polymerizations with a constant off time of 200 ms and variable on times of 10, duty cycles of 10/200, 20/200, and 100/200, respectively. 3

6

duty cycle ratio ( i n this case 1/2) a n d w i t h variations i n pulse widths are shown i n F i g u r e 10. A g a i n , a substantial change i n film composition was noted w i t h respect to carbon functionality w i t h a progressive increase i n C ( I s ) at 287 e V relative to the other C ( I s ) peaks n o t e d at o n - o f f values o f 0.1/0.2, 10/20, a n d 1000/2000 (expressed i n milliseconds). T h e result is


Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993. 230.0

Binding Energy, eV

295.0

285.0

3

6

Figure 10. High-resolution C (is) XPS results obtained with C F at a constant rf on-off duty cycle ratio of 1/2 with variable on-off pulse widths of 0.1 /0.2, 10/20, and 1000/2000 (in milliseconds) as shown. The stacked plots were normalized as in Figure 8.

300.0


32.

SAVAGE ET AL

Puked Radio-Frequency Plasma Discharges

763

again similar to that observed w i t h C F O as shown i n F i g u r e 4, w h e r e the o n - o f f ratio was 1 /20. T h e F T I R spectra obtained i n C F plasma polymerization are also clearly supportive o f m a r k e d changes i n the molecular structure o f the deposited films as functions o f the p u l s e d r f duty cycle e m p l o y e d . T h i s variation i n film composition is illustrated i n F i g u r e 11 for films obtained f r o m C F at r f o n - o f f duty cycles o f 10/20 a n d 10/100 ( i n milliseconds) as curves a a n d b, respectively. A g a i n the single b r o a d b a n d ~ 1200 c m obtained u n d e r 10/20 ( i n milliseconds) conditions, indicative of the presence o f a w i d e range o f C F stretching frequencies (i.e., a r a n d o m i z e d structure), tions i n w h i c h the ratio o f o n - o f f t i m e is reduced. I n v i e w o f the X P S data i n F i g u r e 8, it is clear that the growth i n the peak at ~ 1060 c m can be attributed to the increasing p r o m i n e n c e of C F groups f o r m e d w i t h decreas i n g values o f the o n - o f f r f duty cycle. 3

e

3

3

6

6

- 1


- 1

2

T h e film-deposition rates obtained as a f u n c t i o n of the p u l s e d r f duty cycle variations are shown i n Figures 1 2 - 1 4 . F i g u r e 12 shows the film-de position rates obtained at a constant o n time of 10 ms a n d variable off t i m e u p to 200 ms. I n the case o f C F , the film deposition rate exhibits a sharp m a x i m u m at 20 ms off as the off time is increased. T h i s behavior is quite different f r o m that n o t e d for C F O u n d e r comparable pulsing conditions ( F i g u r e 6). A plot o f deposition rate for a constant off t i m e o f 200 ms a n d variable o n time is shown i n F i g u r e 13. T h i s graph also exhibits a m a x i m u m , observed i n the comparable plot for C F O ( F i g u r e 7). F i g u r e 14 shows the deposition rate as a logarithmic function o f o n t i m e for a series o f runs carried out at a r f duty cycle o n - o f f ratio o f 1/2. A s shown b y these data, pulse times. 3

6

3

e

3

e

T h e variations i n deposition rates p e r absorbed joule w i t h r f duty-cycle changes shown i n Figures 1 2 - 1 4 are not easily rationalized i n terms o f a specific reaction mechanism. T h e initial increasing film deposition rate w i t h increasing off time i n F i g u r e 12 is indicative o f the occurrence o f c o n t i n u e d polymerization d u r i n g the off p e r i o d . H o w e v e r , unlike the results obtained w i t h C F 0 , the deposition rate decreases w i t h increasing off times at off times longer than 20 ms. W e believe that a possible explanation o f this p h e n o m e n o n is that a chain radical polymerization process o f the type 3

6

R + C F ->R(CF ) CF 3

6

2

2

2

(5)

that is analogous to reaction 4 w i t h C F 0 occurs d u r i n g the off p e r i o d . H o w e v e r , the activation energy for reaction 5 may be higher that that o f reaction 4. P u l s e d plasma operations u n d e r conditions i n w h i c h a l o n g off t i m e is operable w i l l p r o d u c e a lower average gas temperature than that 3

6


764



I

4000

!

3500

;

3000

ι

2500

1 2000

Wavenumbers (cm- )

1 1500

!

1000

I

500

1

Figure 11. FTIR absorption spectra obtained from the plasma deposition of C F as a function of the rf duty cycle employed. Curves a and b represent data from runs carried out at rf duty cycles of 10/20 and 10/100 (in milliseconds), respectively. 3

6


32.

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765


400


Ε

0

-t

«

0

1

«

1

1

100

200

300

OFF Time (ms) Figure 12. Film deposition rate obtained during the pulsed rf plasma polymerization of C F for runs at a constant on time of 10 ms expressed as a function of the off time. Deposition rate is expressed in terms of milliangstroms per joule of absorbed energy. 3

6

achieved at short o n times. T h e lower gas temperature, i n t u r n , decreases the extent of film formation achieved d u r i n g the off p e r i o d . T h e initial p o r t i o n o f the curve i n F i g u r e 12 represents contributions to the film growth rate d u r i n g b o t h the o n a n d off periods. It appears that 20 ms off represents

the

o p t i m u m time for polymerization d u r i n g the off p o r t i o n o f the duty cycle. H o w e v e r , as i n the case o f C F O polymerization, it is clear that b o t h 3

e

ablation- a n d reactive-induced surface rearrangements

occur i n the

C F 3

6

p u l s e d plasma polymerization. T h e occurrence of ablation processes is clearly evident i n the film growth rate data shown i n F i g u r e 13 i n w h i c h a decrease i n deposition rate is n o t e d at l o n g o n times. T h e

film

growth rate must

continue to decrease at o n times longer than those shown i n this

figure

because virtually no film is obtained u n d e r C W conditions. T h e occurrence of surface-induced molecular rearrangements

d u r i n g the o n periods is evi

d e n c e d b y the X P S spectral changes observed at constant o n - o f f ratios and varying pulse lengths ( F i g u r e 10). It is clear that there is a progressive shift toward a more highly cross-linked structure [i.e., an increase i n the relative importance o f C ( I s ) w i t h b i n d i n g energy o f 287 eV] w i t h increasing lengths of the on time pulse widths. T h i s surface-induced rearrangement,

which

promotes increased p o l y m e r cross-linking d u r i n g l o n g o n periods w i t h C F 3


6

766


500

400

300


Ε

200

100 Η

300

ON Time (ms) Figure 13. Film deposition rate obtained during the pulsed rf plasma polymerization of C F for runs with a constant off time of200 ms expressed as a function of the on times employed. 3

6

Log ON Time Figure 14. Film deposition rate obtained during the pulsed rf plasma polymerization of C F for runs with a constant on—off ratio of 1 /2 but with variable on-off pulse widths. Deposition rate (in milliangstroms per joule absorbed rf energy) are expressed as a function of the log on time. 3

6


32.

SAVAGE ET AL


767

m o n o m e r , is similar to, b u t significantly more p r o n o u n c e d than the rear rangement observed w i t h C F 0 . 3

6


Discussion Ideally, w e w o u l d like to b e able to predict, a p r i o r i , the molecular composi tion that w i l l b e obtained i n the plasma polymerization o f a given m o n o m e r u n d e r specified reaction conditions. D e s p i t e an impressive n u m b e r o f studies of the plasma polymerization o f numerous molecules, this goal o f molecular structure p r e d i c t i o n o f plasma-derived polymers is simply not available at present. This nonpredictability reflects the complex variety o f processes initiated b y the plasma. Overall, as d o c u m e n t e d i n many literature reports o n C W plasma polymerizations, there is clearly an indiscriminate nature to the plasma polymerization processes. T h e major objective o f this w o r k was to explore the use o f p u l s e d r f plasma discharges as a means b y w h i c h a higher degree o f selectivity a n d control c o u l d b e i n t r o d u c e d w i t h respect to the molecular composition o f p o l y m e r i c materials obtained d u r i n g r f plasma operation. F r o m the results presented i n this chapter, it is clear that a n e w level o f molecular composi tional control o f resulting polymers is i n d e e d attainable as a function o f variations o f the p u l s e d r f duty cycles e m p l o y e d . W e i n t e n d to investigate the generality o f these findings b y extending this study to a w i d e range o f additional monomers. Based o n the results o f this study, several important conclusions are justifiable at this time. T h e first conclusion is that variation i n the p u l s e d r f plasma duty cycle provides an avenue for variation o f surface compositions over significant ranges i n molecular structure. A s demonstrated i n this work, conducted w i t h m i x e d monomers, i n w h i c h the tunability o f molecular surface compositions c o u l d w e l l b e extended over significantly w i d e r ranges. A second conclusion is that the film-forming dynamics i n these p u l s e d plasma depositions are obviously complex a n d represent a variety o f compet ing processes. T h e complexity o f the deposition processes is illustrated i n terms o f the deposition rates p e r joule as a function o f the r f duty cycles as reported, c o u p l e d w i t h the spectroscopic results o f changes i n molecular composition. D u r i n g plasma-on periods, d e p o s i t i o n - a b l a t i o n - r a d i c a l - i n d u c e d surface rearrangements clearly occur. D u r i n g plasma-off periods, additional film deposition is obtained. T h i s plasma-off film formation can b e very significant (e.g., as i n C F O ) a n d it represents polymerization i n w h i c h a higher degree o f specificity is shown w i t h respect to the surface compositions achieved. W e believe it is this difference i n p o l y m e r compositions obtained d u r i n g o n a n d o f f plasma periods that provides the basis f o r tailoring surface structures as demonstrated i n this work. Ultimately, as more results are made 3

e


768


available f r o m p u l s e d r f plasma studies, w e hope to b e able to anticipate a n d predict the nature

o f the surface compositional changes that might b e

achieved as functions o f the plasma duty cycles e m p l o y e d f o r a particular reactant system. F i n a l l y , w e w i s h to acknowledge explicitly that controllability a n d t u n a b i l ity o f the molecular composition o f surfaces represent a n important goal that has obvious applications to a w i d e range o f situations. Practical examples o f the n e e d f o r surface compositional control i n c l u d e such areas as i m p r o v e d biocompatibility o f materials, i m p r o v e d permselectivity o f membranes, better passivation a n d antireflective coatings f o r optical materials, a n d i m p r o v e d h y d r o p h o b i c o r h y d r o p h i l i c surfaces for various applications. Based o n the


spectroscopic

characterization o f the plasma-induced surface modifications

described i n this chapter, it is felt that the p u l s e d r f plasma p o l y m e r i z a t i o n technique demonstrates

great potential w i t h respect to the ultimate goal o f

tailoring surface molecular composition f o r specific applications.

Acknowledgment T h i s material is based, i n part, o n w o r k supported b y the G o v e r n o r s E n e r g y M a n a g e m e n t C e n t e r , State o f Texas E n e r g y Research i n Applications P r o grams u n d e r Contract N u m b e r 0 0 3 6 5 6 - 0 0 5 .

References 1. Symposium on Plasma Polymerization and Plasma Interactions with Polymeric Materials, Division of Polymeric Materials: Science and Engineering, ACS Na tional Meeting, Spring 1990, Boston, Massachusetts. 2. Yasuda, H. Plasma Polymerization; Academic: Orlando, F L , 1985. 3. Chilkoti, Α.; Ratner, B. D.; Briggs, D. Chem. Mater. 1991, 3, 51. 4. Bhuiyan, A. H.; Bhoraskar, S. V. J. Mater. Sci. 1989, 24, 3091. 5. Wydeven, T.; Golub, Μ. Α.; Lerner, N. R. J. Appl. Polym. Sci. 1989, 37, 3343. 6. Evans, J. F.; Prohaska, G. W. Thin Solid Films 1984, 118, 171. 7. Lopez, G.; Ratner, B. ACS Polym. Mater. Sci. Eng. 1990, 62, 14. 8. O'Kane, D. F.; Rice, D. W. J. Macromol. Sci., Chem. 1976, A10(3), 567. 9. Yasuda, H.; Hsu, T. J. Polym. Sci., Chem. Ed. 1977, 15, 81. 10. Nakajima, K.; Bell, A. T.; Shen, M. J. Appl. Polym. Sci. 1979, 23, 2627. 11. Clark, D. T.; Shuttleworth, D. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 27. 12. Wang, D.; Chen, J. J. Appl. Polym. Sci. 1991, 42, 233. 13. JANAF Thermochemical Tables; 2nd ed.; U.S. Department of Commerce, Na tional Bureau of Standards: Washington, D C , 1971. RECEIVED for review July 15, 1991. ACCEPTED revised manuscript September 9, 1992.


Spectroscopic Characterization of Films Obtained in Pulsed Radio

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